Finasteride is a prescription drug sold in Canada under two brand names: Propecia, a one-milligram dose, is used to treat male-pattern hair loss, while Proscar, a five-milligram pill, is for benign prostatic hyperplasia or BPH — a non-cancerous enlargement of the prostate. Generic versions of both are also sold.

Propecia is designed to treat male pattern baldness, the most common form of baldness where the hair recedes on either side of the head. ((Ian Waldie/Reuters))

"Male breast cancer has been reported in a small number of patients worldwide with both the one-milligram and five-milligram formulations of finasteride," Health Canada said Thursday.

"Most of the reports have been in association with the five-milligram formulation. Based on the currently available evidence, it is not known with certainty whether finasteride can cause breast cancer, nor can this possibility be ruled out at this point in time."

Up to March 31, Health Canada received two reports of male breast cancer involving the five-milligram dose and no reports involving the one-milligram dose, the department said in an email.

Labels for Propecia and Proscar — both made by U.S. pharmaceutical giant Merck — as well as several generic finasteride products have already been updated to include information on the potential risk of male breast cancer, and the other generic drugs will also be updated, the department said.

Patients taking finasteride should report any changes in their breasts to their doctor. Changes might include breast enlargement, lumps, tenderness, pain or nipple discharge.